Chemical delivery systems and methods of delivery

ABSTRACT

The present invention relates to chemical delivery systems and methods for delivery of liquid chemicals. In one embodiment, the present invention relates to systems having multi-reservoir load cell assemblies for delivering chemicals used in the semiconductor industry. In one embodiment, the present invention provides a multi-reservoir load cell assembly, including a controller, a buffer reservoir, a main reservoir, one or more load cells, coupled to the assembly and to the controller, operable to weigh the liquid in the reservoir(s), a plurality of supply lines, each supply line having a valve and connecting one of the supply containers to the main reservoir, and a gas and vacuum sources for withdrawing the liquid from the assembly when demanded by the controller and for refilling the assembly from the supply containers.

BACKGROUND

The present invention relates generally to systems and methods fordelivering of liquid chemicals, and more particularly, to systems andmethods for delivery of liquid chemicals in precise amounts using logicdevices and multi-reservoir load cell assemblies.

The present invention has many applications, but may be best explainedby considering the problem of how to deliver photoresist to siliconwafers for exposure of the photoresist in the process ofphotolithography. To form the precise images required, the photoresistmust be delivered in precise amounts on demand, be free of bubbles, andbe of precise uniform thickness on the usable part of the wafer. Theconventional systems have problems as discussed below.

As shown in FIG. 1, a representative conventional photoresist deliverysystem includes supply containers 100, 102, typically bottles, whichsupply photoresist to a single-reservoir 104 by line 117, which isconnected to supply lines 106, 108 monitored by bubble sensors 110, 112and controlled by valves V1 and V2. The bottom of the reservoir isconnected to a photoresist output line 114 to a track tool (not shown)which dispenses photoresist on the wafer. The space above thephotoresist in the reservoir 104 is connected to a gas line 118 which,based on position of a three-way valve V3, either supplies nitrogen gasto the reservoir 104 from a nitrogen manifold line 126, regulated byneedle valve 120, or produces a vacuum in the reservoir 104. To sensethe level of the photoresist in the reservoir 104, the system employs anarray of capacitive sensors 122 arranged vertically on the walls of thereservoir 104. A two-way valve V4, located between the nitrogen gasmanifold and the inlet of a vacuum ejector 124, supplies or cuts offflow of nitrogen to the vacuum ejector 124.

The photoresist delivery system must be "on-line" at all times so thetrack tool can dispense the photoresist as required. Many of thephotoresist delivery systems attempt to use the reservoir to provide anon-line supply of photoresist to the track tool, but the photoresistdelivery system must still refill the reservoir on a regular basis whichis dependent on timely replacement of empty supply containers.Otherwise, the track tool will still fail to deliver the photoresistwhen demanded.

During dispense mode, when photoresist is withdrawn by the track toolfrom the reservoir 104, the valve V3 permits the nitrogen to flow fromthe nitrogen manifold to the reservoir 104 to produce a nitrogen blanketover the photoresist to reduce contamination and to prevent a vacuumfrom forming as the photoresist level drops in the reservoir. Once thephotoresist in the reservoir 104 reaches a sufficiently low level thesystem controller (not shown) initiates refill mode, where a set ofproblems arise.

During refill mode, the valve V4 is activated so that nitrogen flowsfrom the manifold line 126 to the vacuum ejector 124 which produces alow pressure line 170 thereby producing a low pressure space above thephotoresist in the reservoir 104. The bubble sensors 110, 112 monitorfor bubbles in the supply lines 106, 108, presumed to develop when thesupply containers 100, 102, become empty. If, for example, the bubblesensor 110 detects a bubble, the controller turns off the valve V1 tosupply container 100 and the valve V2 opens to supply container 102 tocontinue refilling the reservoir 104. However, bubbles in the supplyline 106 may not mean supply container 100 is empty. Thus, not all ofthe photoresist in supply container 100 may be used before the systemswitches to the supply container 102 for photoresist. Thus, although theconventional system is intended to allow multiple supply containers toreplenish the reservoir when needed, the system may indicate that asupply container is empty and needs to be replaced before necessary.

If the supply container 100 becomes empty and the operator fails toreplace it and the system continues to operate until the supplycontainer 102 also becomes empty, the reservoir 104 will reach acritical low level condition. If this continues, bubbles may be arisedue to photoresist's high susceptibility to bubbles; if a bubble,however minute, enters the photoresist delivered to the wafer, animperfect image may be formed in the photolithography process.

Further, if the pump of the track tool, connected downstream of thechemical output line 114, turns on when the reservoir is refilling, thepump will experience negative pressure from the vacuum in thesingle-reservoir pulling against the pump. Several things can happen ifthis persists: the lack of photoresist delivered to the track tool maysend a false signal that the supply containers are empty, the pump canfail to deliver photoresist to its own internal chambers, lose its primeand ability to adequately dispense photoresist, and the pump can evenoverheat and burn out. The result of each scenario will be the tracktool receives insufficient or even no photoresist, known as a "missedshot," which impacts the yield of the track tool.

The present invention addresses these problems as well as avoids wasteof expensive photoresist, provides a friendly user interface depictingthe amount of photoresist remaining in the supply containers, andreduces system capital and operating costs. If, for example, the amountof photoresist in the supply containers cannot be seen, the presentinvention permits the interface to be provided at a distance byconventional computer network capabilities and the electronics provided.

SUMMARY OF THE INVENTION

The present invention relates to systems using controllers or logicdevices and multi-reservoir load cell assemblies for precision deliveryof liquid chemicals. It also relates to methods of delivering liquidchemicals from supply sources to processes such that the presentinvention accurately accounts and adjusts for the dynamic supply and useof the liquid chemical to meet process requirements. Finally, thepresent invention provides multi-reservoir load cell assemblies formonitoring, regulating, and analyzing the liquid supply available to aprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a chemical delivery system using a single-reservoirand bubble sensors on the supply lines leading to the single-reservoir.

FIG. 2A is a front cross-section of a first embodiment of themulti-reservoir load cell assembly of the present invention.

FIG. 2B is a top view of the first embodiment of the multi-reservoirload cell assembly.

FIG. 3, a piping and instrument diagram, illustrates embodiments of thechemical delivery system including the multi-reservoir load cellassemblies of FIGS. 2A-2B or 4A-4B.

FIG. 4A is a front cross-section of a second embodiment of themulti-reservoir load cell assembly.

FIG. 4B is a side cross-section of the second embodiment of themulti-reservoir load cell assembly.

FIG. 5A is a front cross-section of a third and sixth embodiment of themulti-reservoir load cell assembly.

FIG. 5B is a side cross-section of the third and sixth embodiment of themulti-reservoir load cell assembly.

FIG. 6, a piping and instrument diagram, illustrates embodiments of thechemical delivery system including the multi-reservoir load cellassemblies of FIGS. 5A-5B or 11A-11B.

FIG. 7A is a front cross-section of a fourth embodiment of themulti-reservoir load cell assembly.

FIG. 7B is a side cross-section of the fourth embodiment of themulti-reservoir load cell assembly.

FIG. 8, a piping and instrument diagram, illustrates an embodiment ofthe chemical delivery system including the multi-reservoir load cellassembly of FIGS. 7A-7B.

FIG. 9A is a front cross-section of a fifth embodiment of themulti-reservoir load cell assembly.

FIG. 9B is a side cross-section of the fifth embodiment of themulti-reservoir load cell assembly.

FIG. 10, a piping and instrument diagram, illustrates an embodiment ofthe chemical delivery system including the multi-reservoir load cellassembly of FIGS. 9A-9B.

FIG. 11A is a front cross-section of a seventh embodiment of themulti-reservoir load cell assembly.

FIG. 11B is a side cross-section of the seventh embodiment of themulti-reservoir load cell assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the first embodiment, the present invention includes amulti-reservoir load cell assembly 200 as shown in FIGS. 2A-2B. Theassembly 200 can be part of the system shown in FIG. 3, and can replacethe problematic single-reservoir 104 and bubble sensors 110, 112 of FIG.1.

In this embodiment, the assembly 200, constructed of Teflon, SST or anychemical compatible material, includes an upper compartment 202, a mainreservoir 206, and a buffer reservoir 208, all in an outer housing 212.The buffer reservoir 208 is sealed from the main reservoir 206 by aseparator 209, and an o-ring seal 211 seals the perimeter of theseparator 209 to the outer housing 212. The separator 209 uses a centerconical hole 250 that allows an internal sealing shaft 204 to form aliquid and gas-tight seal with the separator 209. The separator 209forms a liquid and gas-tight seal to the pneumatic tube 215 with ano-ring seal 210. The main reservoir 206 contains a middle sleeve 214that forms a rigid separation between the separator 209 and thereservoir cap 205. The perimeter of reservoir cap 205 seals against theinternal surface of the outer housing 212 with the use of an o-ring 203.The reservoir cap 205 seals against the internal sealing shaft 204, thechemical input tube 217, and the pneumatic tubes 215 and 218 with a setof o-ring seals 207, 220, 222, and 224 (hidden, but location shown inFIG. 2B), respectively. Mounted to the reservoir cap 205 is a spacer 244which also mounts to the pneumatic cylinder 226. The reservoir cap 205is held in position by the upper sleeve 233 and the middle sleeve 214.The outer Teflon reservoir top 201 is bolted to the outer housing 212and forms a mechanical hard stop for the upper sleeve 233 and thepneumatic cylinder 226. Pneumatic air lines for the pneumatic cylinder226 penetrate the outer Teflon reservoir top 201 through the clearancehole 260.

It should be clear that the present invention is not limited to thedelivery of photoresist on silicon wafers. For example, although theinvention shows advantages over the conventional system in thisenvironment, the systems of the present invention can deliver otherliquid chemicals for other types of processes, such as the delivery ofdeveloper or chemical mechanical polishing slurries. Because the noveltyof the present invention extends beyond the nature of the chemical beingdelivered, the following description refers to the delivery of chemicalsto avoid a misunderstanding regarding the scope of the invention.

As shown in FIG. 3, the multi-reservoir load cell assembly 200 shown inFIGS. 2A-2B is suspended on and weighed by a load cell 412, preferablysuch as a Scaime load cell model no. F60X10C610E and a programmablelogic controller (PLC) 330, preferably such as the Mitsubishi FX2N, acomputer, or another conventional logic device determines the volume ofthe chemical in the assembly 200 from the load cell weight and thespecific gravity of the chemical. As chemical from line 217 is drawninto the main reservoir 206, the load cell 412 outputs a small mV analogsignal 324 proportional to the weight on the load cell 412. In oneembodiment, an ATX-1000 signal amplifier 326 boosts the small signal 324to the 4-20 millivolt range and sends it to an analog-to-digitalconverter 328, such as the Mitsubishi FX2N4-AD, and the output digitalsignal 332 is sent to the PLC 330. The PLC 330 can be rapidly programmedby conventional ladder logic. During withdrawal of the chemical, theweight of the assembly 200 decreases until the software set point of thePLC 330 is reached.

As further shown in FIG. 3, the PLC 330 may control valves V1-V5 using24 DC Volt solenoid actuated valves, and activate them by an output cardsuch as the Mitsubishi FX2N. Each solenoid valve, when opened, allowspressurized gas from regulator 2 such as a VeriFlow self-relievingregulator, to the pneumatically operated valves V1-V5 to open or closethe valves. The sequence of operation of the first embodiment isprogrammed in the PLC 330 so the components shown in FIGS. 2A-2B and 3work as described below.

Once the chemical drops to a certain level, the PLC 330 triggers thesystem shown in FIG. 3 to begin an automatic refill sequence using themulti-reservoir load cell assembly 200 of FIGS. 2A-2B as follows:

a) A blanket of preferably low pressure, e.g., one psi inert gas iscontinuously supplied by the regulator 1, such as a Veriflowself-relieving regulator, to the main reservoir 206 by the pneumatictube 218.

b) The internal sealing shaft 204 is lifted by the pneumatic cylinder226, thereby sealing the buffer reservoir 208 from the main reservoir206.

c) Once the buffer reservoir 208 is sealed, the main reservoir 206 isevacuated to a vacuum of approximately 28 inches of mercury. As shown inFIGS. 2A-2B, the pneumatic tube 218 from the main reservoir 206 connectsto the output side of a three-way valve V4. Valve V4 is actuated so thatthe tube 218 communicates with the line 316 connected to the vacuumejector 324 as shown in FIG. 3. The vacuum ejector 324 is powered bycompressed gas which is directed to it by the two-way valve V5. Oncevalve V5 is on, it allows compressed gas to pass through and the vacuumejector 324 develops about 28 inches of mercury (vacuum) through theline 316 communicating with the main reservoir 206.

d) The vacuum is isolated from the buffer reservoir 208 which has aninert gas slight blanket above it and continues to supply chemical tothe process or tool without exposing the chemical being delivered to thetool to negative pressure or a difference in pressure.

e) The vacuum generated in the main reservoir 206 creates a low pressurechemical line that is connected to the valves V1 and V2. Assuming thatvalve V2 opens, the low pressure line 217 causes chemical from thesupply container 102 to flow into the main reservoir 206. During thisperiod of time the main reservoir 206 refills with chemical until adetermined full level is achieved.

f) The full level is determined by use of the load cell 412 and weightcalculations performed by the PLC 330. For example, one preferredembodiment uses a buffer reservoir 208 with a volume capacity of 439cubic centimeters (cc) and a main reservoir 206 with a capacity of 695cc. Using the specific gravity of the chemical, the PLC 330 calculatesthe volume that the chemical occupies. The PLC 330 then begins a refillsequence once the chemical volume reaches or falls below 439 cc. Therefill stops once the chemical volume reaches 695 cc. This sequenceallows nearly all of the 439 cc of the chemical in the buffer reservoir208 to be consumed while refilling the main reservoir 206 with the 695cc of chemical and prevents overflow of the main reservoir 206 orcomplete evacuation of chemical from the buffer reservoir 208.

g) Once the main reservoir 206 has refilled, the valve V5 is turned off,thereby stopping gas flow to and vacuum generation by the vacuum ejector324. The three-way valve V4 is then switched so that the inert gas line218 communicates with the main reservoir 206 and an inert gas blanket isagain formed over the chemical in the main reservoir 206 at the samepressure as the buffer reservoir 208, since both lines 218, 215 receivegas from the same inert gas manifold 318 (see FIG. 3). Also, the valveV2 is closed which now isolates the supply container 102 from the mainreservoir 206.

After the main reservoir 206 is full of chemical with an inert gasblanket above, the internal sealing shaft 204 is lowered and allowschemical from the main reservoir 206 to flow into the buffer reservoir208. Eventually, the buffer reservoir 208 completely fills along with amajority of the main reservoir 206. The pneumatic tube 215 connectingthe buffer reservoir 208 fills with chemical until the chemical in thetube 215 reaches the same level as the main reservoir 206, because thepressures in both reservoirs are identical. The internal sealing shaft204 remains open until it is determined, to once again, refill the mainreservoir 206.

Because the first embodiment uses load cells instead of bubble sensorsfor determining the amount of chemical in the supply containers, thepresent invention provides a number of very useful features. One canaccurately determine in real-time the chemical remaining in the supplycontainers. If the supply containers are full when connected to thesystem, the PLC can easily calculate the chemical removed (and added tothe multi-reservoir load cell assembly) and how much chemical remains inthe supply containers. This information can be used to provide agraphical representation of the remaining amount of chemical in thecontainers. A second feature is that the PLC can determine preciselywhen a supply container is completely empty by monitoring the weightgain within the system. If the weight of the reservoir does not increaseduring a refill sequence then the supply container is inferred to beempty. This causes the valve for the supply container to be closed andthe next supply container to be brought on line. A related third featureis the load cell technology provides the ability to accurately forecastand identify the trends in chemical usage. Since the exact amount ofchemical is measured coming into the reservoir the information can beeasily electronically stored and manipulated and transmitted.

A second embodiment of the multi-reservoir load cell assembly 400 shownin FIGS. 4A-4B, includes a buffer reservoir 408, fastened and sealed bythe o-rings 411 to the bottom cap 410. The output chemical flows throughtube connection 401. Connected to the buffer reservoir 408 are apneumatic tube 415, a chemical valve 407, a load cell separator 413, andthe load cell 412. The load cell 412 is securely bolted to the bufferreservoir 408 and the other side is securely bolted to a rigid member(not shown) not part of the multi-reservoir load cell assembly 400. Theouter sleeve 404 slips around the buffer reservoir 408 and rests againstthe bottom cap 410. The outer sleeve 404 is machined to allow the loadcell 412 to pass through it unencumbered. End 405 of the valve 407connects to the main reservoir 406 and the other end 409 connects tobuffer reservoir 408. The main reservoir 406 is encapsulated and sealed,by o-rings in the upper cap 403. The upper cap 403 incorporates astepped edge along its periphery to secure the outer sleeve 404 to it.Pneumatic line 418 and chemical input line 417 are secured to the uppercap 403. The outer sleeve 404 provides the mechanical strength for theseparate reservoirs 406 and 408.

The multi-reservoir load cell assembly shown in FIGS. 4A-4B, and used inthe system of FIG. 3, is similar to the first embodiment with thefollowing notable differences:

a) Valve 407 provides control of the fluid path between the mainreservoir 406 and the buffer reservoir 408.

b) The outer sleeve 404 provides the mechanical support to form therigid assembly that supports the main reservoir 406 as well as thebuffer reservoir 408.

A third embodiment of the multi-reservoir load cell assembly shown inFIGS. 5A-5B, employs two reservoirs 506, 508 spaced apart from eachother but connected by a flexible fluid line 516. The third embodimentuses many of the previous components shown in FIGS. 4A-4B, except: (i)it does not use an outer sleeve 404; (ii) the buffer reservoir 508 isnot mechanically suspended from the main reservoir 506; and (iii) theload cell spacer 513 and the load cell 512 are fastened to the bottom ofthe main reservoir 506.

The third embodiment operates like the second embodiment except the loadcell 512 only measures the volume of chemical in the main reservoir tank506 as shown in FIGS. 5A-5B and 6. The advantage of the third embodimentis the precise amount of chemical brought into the main reservoir 506 isalways known and the PLC does not have to infer the amount of chemicalthat was removed from the buffer reservoir 508 during a refilloperation. The third embodiment can be used in the system of FIG. 6 withthe control system (i.e., PLC, A/D, signal amplifier, etc.) of FIG. 3.Note, in the application, the lead digit of the part numbers generallyindicates which drawing shows the details of the part, while thetrailing digits indicate that the part is like other parts with the sametrailing digits. Thus, the buffer reservoir 206 and the buffer reservoir306 are similar in function, and found in FIGS. 2A and FIG. 3A,respectively.

A fourth embodiment of the multi-reservoir load cell assembly 700 shownin FIGS. 7A-7B, employs the same components as the third embodiment,however, a second load cell 722 is attached to the buffer reservoir 708.The assembly 700 is preferably used with the system of FIG. 8 with thecontrol system of FIG. 3 with additional components for the second loadcell.

The fourth embodiment of the multi-reservoir load cell assembly 700shown in FIGS. 7A-7B, operates much like the second embodiment exceptthat the load cell 712 only measures the chemical in the main reservoir706 and the load cell 722 only measures the chemical in the bufferreservoir 708. The advantage here is the buffer reservoir 708 isconstantly monitored so if the downstream process or tool suddenlyconsumes large amounts of chemical during a refill cycle, the system canstop the refill cycle short to bring chemical into the buffer reservoir708 from the main reservoir 706 to prevent the complete evacuation ofchemical from the buffer reservoir 708.

A fifth embodiment of the multi-reservoir load cell assembly 900 shownin FIGS. 9A-9B uses the same components as the third embodiment, exceptthe load cell 912 is attached to the buffer reservoir 908 instead of themain reservoir 906. The fifth embodiment is preferably used in thesystem depicted in FIG. 10 with the control system (i.e., PLC, A/D,signal amplifier, etc.) shown in FIG. 3.

Functionally, the fifth embodiment of the multi-reservoir load cellassembly 900 operates the same as the second embodiment, the onlydifference is the load cell 912 only weighs the chemical in the bufferreservoir 908.

As the process or tool consumes the chemical, the weight of the bufferreservoir 908 remains constant until the main reservoir 906 also becomesempty. Then the weight in the buffer reservoir 908 will start todecrease, indicating that the main reservoir 906 needs to be refilled.At this point the main reservoir 906 is refilled for a calculated periodof time. During this sequence the buffer reservoir 908 decreases untilthe main reservoir 906 has been refilled and the valve 907 has beenreopened between the two reservoirs 906, 908.

A sixth embodiment uses the same components of third embodiment shown inFIG. 5A-5B. The only notable difference is that the inert gas blanket(see FIG. 6) of approximately one psi is increased to approximately 80psi (more or less depending on the type of chemical). The increasedinert gas pressure enables the sixth embodiment to pressure dispense thechemical at a constant output pressure which remains unaffected evenduring the refill cycle. This method would allow very precise non-pulsedoutput flow of the chemical. This may be a highly critical feature in anultra high purity application that pumps the chemical through a filterbank. Any pulsation of the chemical can cause particles to be dislodgedfrom the filter bank into the ultrapure chemical output flow.

A seventh embodiment uses the same components as the third embodimentwith additional components shown in FIGS. 11A-11B, including a mainreservoir 1106, a buffer reservoir 1108, a second chemical input line1119 added to the main reservoir 1106 through the valve 1122, a valve1123 added to the chemical input line 1117, and a stir motor 1120 and animpeller assembly 1121.

Functionally, the seventh embodiment operates the same as the thirdembodiment with the added capability of mixing two chemicals in preciseproportions before transferring the mixture to the buffer reservoir1108. The chemical can be drawn into the main reservoir 1106 throughopen valve 1123 and the chemical input line 1117 and weighed by the loadcell 1112. When the proper amount has been drawn into the main reservoir1106, the valve 1123 is closed and the valve 1122 is opened to allow thesecond chemical to enter the main reservoir 1106. When the proper amounthas been drawn into the main reservoir 1106, the valve 1122 is closedand the chemicals are blended via the stir motor 1120 and impellerassembly 1121. The stirring of the chemicals can be initiated at anytime during the above sequence. Once the mixing is complete, the valve1107 opens to allow the chemical to transfer to the buffer reservoir1108, which is also connected to gas line 1115. This is an ideal way tomix time sensitive chemistries and maintain a constant, non-pulsedoutput of the blended chemicals.

In review, the present invention provides at least the followingbenefits. The output chemical can be maintained at a constant pressure.A track tool never experiences a low pressure chemical line that couldprevent a dispense sequence from occurring, therefore the yield of thetrack tool is increased. A multitude of containers and sizes can beconnected to the reservoir system as chemical supply containers. If thefluid volume of the supply containers are known before they areconnected, the computer can calculate very accurately the amount ofchemical that has been removed from the container and therefore presentthe information to a display for a visual, real time indication of theremaining amount of chemical. The graphical interface communicates tothe operator at a "glance" the condition of the supply containers. Theload cells can determine when the supply container is completely emptysince there will not be a continued weight increase during a refillsequence. This indicates the supply container is empty and that anothercontainer should be brought on line. In one embodiment, data logging ofchemical usage can be provided since the chemical in the reservoir(s) iscontinuously and accurately weighed by load cell(s) which give an inputsignal to the PLC or other logic device which outputs real time,accurate information as to the amount of chemical available in thereservoir. The load cell is an inherently safe sensing device sincefailure is indicated by an abnormally large reading or an immediate zeroreading, both of which cause the PLC or other logic device to trigger analarm. The invention can also prevent bubbles that occur during a supplycontainer switching operation from passing through to the outputchemical line, can provide constant, non-varying pressure dispense withmultiple supply containers, can refill itself by vacuum or by pumpingliquid to refill the reservoir or refill with different chemicals atprecise ratios and mix them before transferring the mixture to thebuffer reservoir, which may be important for time dependent, veryreactive chemistries.

What is claimed:
 1. A liquid chemical delivery system for use with aplurality of supply containers, comprising:a controller; amulti-reservoir load cell assembly, including, a buffer reservoir with achemical output, a main reservoir, a reservoir valve connecting thebuffer reservoir to the main reservoir, and a load cell, coupled to theassembly and to the controller, operable to weigh the liquid in thereservoir assembly; a plurality of supply lines, each having a valvecoupled to the controller and to one of the supply containers to themain reservoir; and means for withdrawing the liquid from the chemicaloutput, opening and closing the reservoir valve, and refilling the mainreservoir from the supply containers when demanded by the controllerbased on signals from the load cell.
 2. The system of claim 1, whereinthe means for withdrawing the liquid from the output and for refillingthe main reservoir from the supply containers, includes a gas lineconnected to the main reservoir, a gas source connected to the gas line,a vacuum source, a vacuum line connecting the vacuum source to the gasline, a gas valve connecting the gas line, the gas source, and thevacuum source, wherein the controller opens the gas valve to permit gasto flow from the gas source to the main reservoir when liquid iswithdrawn from the reservoir and to generate a vacuum in the mainreservoir when the reservoir is refilled from the supply containers. 3.The system of claim 1, wherein the controller closes the bufferreservoir from the main reservoir when the reservoir is refilled andliquid is withdrawn such that the buffer reservoir undergoes no negativepressure from the vacuum in the main reservoir.
 4. A chemical deliverysystem, comprising:a multi-reservoir load cell assembly comprising amain reservoir capable of fluid communication with a buffer reservoirand a first load cell weighing the assembly and generating an outputsignal indicative of the weight; and means, responsive to the outputsignal, for adjusting the pressure in the main reservoir and in thebuffer reservoir, for calculating the amount of chemical in theassembly, and for controlling fluid communication between the mainreservoir from the buffer reservoir.
 5. The system of claim 4, whereinthe first load cell generates an output signal proportional to theweight of the chemical in the main reservoir.
 6. The system of claim 4,wherein the first load cell generates an output signal proportional tothe weight of the chemical in the buffer reservoir.
 7. The system ofclaim 4, 5, or 6, wherein the means responsive to the output signalincludes means for refill of the assembly with the chemical when theweight of chemical drops to a predetermined amount by the followingsequence:a) isolating the main reservoir from the buffer reservoir, b)reducing the gas pressure in the main reservoir to draw the chemicalinto the main reservoir until the chemical rises to a predeterminedamount, c) increasing the gas pressure in the main reservoir, and d)opening communication between the main reservoir and the bufferreservoir to allow the chemical in the main reservoir to flow into thebuffer reservoir.
 8. The system of claim 5, further comprising a firstsupply container connected to the main reservoir, a second supplycontainer connected to the main reservoir, a first valve associated withthe first supply container, a second valve associated with a secondsupply container, and wherein the means responsive to the output signalmonitors the first output signal and shuts and opens the first valvebased on the value of the first output signal.
 9. The system of claim 5,further comprising a first supply container connected by a first line tothe main reservoir, a second supply container connected by a second lineto the main reservoir, a first valve on the first line, a second valveon the second line, wherein the logic device monitors the first outputsignal and shuts and opens the first valve based on the value of thefirst output signal.
 10. A liquid chemical delivery system, comprising:amulti-reservoir load cell assembly, including a main reservoir with agas inlet, a liquid inlet, and a liquid outlet, and a buffer reservoirwith a gas inlet and a liquid inlet; at least one load cell to weigh thechemical contained in the assembly and generate an output signalindicative of the weight; a first valve and a second valve associatedwith the main reservoir liquid inlet; a third valve connecting the mainreservoir liquid outlet and the buffer reservoir liquid inlet; a fourthvalve connecting the main reservoir gas inlet and the buffer reservoirgas inlet; means, responsive to the output signal, for refilling thebuffer reservoir with chemical when the output signal indicates chemicalin the buffer reservoir has dropped to a predetermined level,comprising:closing the third valve to isolate the main reservoir fromthe buffer reservoir, opening the fourth valve to evacuate the mainreservoir to draw liquid chemical into the main reservoir until theliquid chemical rises to a predetermined level, opening the valvebetween the main reservoir and the buffer reservoir to allow the liquidchemical in the main reservoir to flow into the buffer reservoir.
 11. Aliquid chemical delivery system coupled to one or more supplycontainers, comprising:a multi-reservoir load cell assembly, including amain reservoir, a buffer reservoir, and at least one load cell; a logicdevice coupled to the load cell(s) providing output signals toactuate:means for sealing and unsealing the buffer reservoir from themain reservoir, a vacuum generator for evacuating the main reservoir todraw liquid chemical from the supply container(s) to the main reservoir;and a gas source supplying the main reservoir to blanket the liquidchemical in the main reservoir.
 12. The system of claim 11, wherein themeans for sealing and unsealing includes a shaft or valve disposedbetween the main reservoir from the buffer reservoir.
 13. The system ofclaim 11, wherein the assembly includes a first load cell and a secondload cell generating first and second signals indicating the weight ofthe main and buffer reservoirs to the logic device.
 14. The system ofclaim 11, wherein the logic device monitors the weight of the bufferreservoir so that if the rate of liquid chemical consumption will emptythe buffer reservoir before the refill is complete, the system can stoprefill and/or transport liquid chemical from the main reservoir to thebuffer reservoir to prevent emptying the buffer reservoir.
 15. Thesystem of claim 11, wherein the means for refill include means forflexibly connecting the buffer reservoir to a main valve at the mainreservoir and suspending the buffer reservoir from the main reservoir.16. The system of claim 13, further comprising means for starting refillwhen the second load cell detects the weight of the buffer reservoir isdecreasing which indicates the main reservoir is empty.
 17. A liquidchemical delivery system adapted for one or more supply containers,comprising:a multi-reservoir load cell assembly, including a mainreservoir, a buffer reservoir, and at least one load cell; a logicdevice coupled to the load cell(s) providing output signals toactuate:means for sealing and unsealing the buffer reservoir from themain reservoir, a vacuum generator communicating with the main reservoirfor evacuating the main reservoir to draw liquid chemical from thesupply container(s) into the main reservoir; and a gas sourcecommunicating with the main reservoir to blanket the liquid chemical inthe main reservoir.